1. Field of the Invention
This invention pertains to a method for producing metal/insulator composite materials (cermets).
2. Prior Art
Composite two phase materials consisting of finely dispersed mixtures of immiscible metals, materials with relatively high conductivity, and insulators, materials with substantially lower conductivity, have been studied extensively and are finding an increasing use in a variety of applications. These applications include film resistors for microelectronics fabrication, ferromagnets for magnetic core materials, optical materials for solar energy conversion systems and piezoresistive materials for pressure measurement. The microstructure and physical properties (e.g. electrical, optical, magnetic) of these composite materials, also known as cermets, depend very sensitively on the method of preparation.
Conventionally, cermet films can be prepared either by co-sputtering or co-evaporation of immiscible metals and insulators or by screen-printing of inks. Such inks have insulator and metal particles suspended in a viscous organic fluid and, after screen-printing, are dried and fired. A widely used method for preparing cermet films is by sputtering from a composite target 10 as shown schematically in FIG. 1A. A typical composite target includes a disc where part of the disc is composed of a metallic component and part is composed of an insulator 12. A plasma discharge in a low pressure gas (e.g. argon) is supported between target 10 and a substrate 13 by the application of a radio frequency (RF) electric field. The gas ions impinge on target 10 and sputter off target material. Both target 10 and substrate 13 are held stationary. The deposition rate of material from each component of target 10 is non-uniformly distributed across substrate 13.
Also, U.S. Pat. No. 4,021,277 to Shirn et al teaches forming a thin metal alloy film of nickel chromium by rotating a substrate under a plurality of sputtering cathodes. The substrate is alternately disposed directly beneath a chromium-plated target and a nickel chromium target. The sputtering rate of each target is individually controlled by adjusting an associated power supply level.
Determining the volume fraction of metal in the sputtered composite film requires measurement of the sputter yield of the two components of the composite target, shown in FIG. 1A. Given these yields and the dependence of the thickness of the film on a substrate location parameter, s, (see FIG. 1A), Hanak's model (see J. J. Hanak, H. W. Lehman, and R. K. Wehner, J. Appl. Phys. 43, 1666 '72) predicts the volume fraction of metal, f, as a function of s. The target configuration of FIG. 1A allows the preparation in a single run of samples with metal volume fraction, f, that spans a large part of the range from 0 to 1. On the other hand, the size of a sample with a given, uniform composition is very small.
A more uniform composition over a large area may be obtained with a composite target configuration 15 indicated in FIG. 1B. Composite films prepared by co-sputtering show a strong dependence of the composite particle size on the composite metal volume fraction.
The composite target method produces metal/ insulator composites which for a metal volume fraction, f, less than about 0.5-0.6, have metal particles with a wide range of shapes and a broad size distribution. Furthermore, the average size distribution is found to depend on the metal volume fraction as shown in FIG. 2. This means that the particle size cannot be changed independently of the volume fraction.
Since the physical properties of the two-phase metal and insulator composites, i.e. cermets, depend not only on the metal volume fraction but also on the average metal particle size and shape, the particle size spread and the type of distribution of the particles in the insulator, it would be desirable to have a method that provides uniform metal particle size and shape, provides uniform metal particle distribution in the insulator and allows control of particle size independently of metal volume fraction. These are some of the problems this invention overcomes.